This invention relates to epitaxial silicon films and to a method for improving their quality. In a more particular aspect, this invention concerns itself with improving the quality of silicon films previously deposited on insulating substrates.
The recent interest in the use of integrated circuit technology has spawned a considerable research effort in an attempt to develop high quality materials for use in integrated circuits requiring radiation-hard, high speed and high density properties. Amongst the materials developed for use in integrated circuits are single crystal silicon films. Generally, these films are deposited onto insulating substrates using convention epitaxial deposition techniques. For example, the chemical vapor deposition (CVD) of silicon as an epitaxial layer onto a sapphire substrate (SOS) has provided a state-of-the art material useful in fabricating large scale integrated circuits. These silicon coated substrates are generally available as 3 to 4 inch diameter wafers. Unfortunately, the silicon coated sapphire substrates (SOS) do not possess the requisite properties needed for high speed circuitry operations. Although 0.5-0.6 .mu.m thick SOS films are routinely used for fabricating large-scale integrated circuits, using metal-oxide-semiconductor (MOS) and field-effect transistors (FETs) with dimensions down to, and sometimes below 2 .mu.m, carrier mobilities are limited by the extended lattice defects and .perspectiveto.-4.times.10.sup.-3 compressive in-plane strain present in these SOS films. Typical field-effect electron and hole mobilities in CVD SOS are .mu..sub.eFE =400-500 cm.sup.2 /Vs and .mu. .sub.hFE =150-200 cm.sup.2 /Vs, respectively. These values are below those obtained bulk silicon, and are a limiting factor in circuit speed. In addition, the sub-threshold backside leakage current in SOS devices is relatively high, due to the highly defective near-interface region in the silicon films. The compressive strain present in SOS films at room-temperature is attributed to the higher thermal expansion coefficient of sapphire, compared to that of silicon. During cooling from the silicon deposition temperature, typically 900.degree.-1000.degree. C., to room-temperature, the thin silicon film is forced to follow the larger contraction of the much thicker (18 mil or 0.46 mm) sapphire wafer. The measured strain in SOS is about 85% of the expected value. The remaining 15% is presumably relieved into the observed stacking faults, microtwins, and dislocations. The concentration of these extended lattice defects is highest at the Si/Al.sub.2 O.sub.3 interface, and decreases away from it. However, the silicon surface is not free of these defects, and its crystalline quality is substantially poorer than that of bulk silicon, even for films as thick as 1 .mu.m. There are additional drawbacks of using sapphire (Al.sub.2 O.sub.3) as a substrate for silicon. First, it is structurally mismatched to silicon, since Al.sub.2 O.sub.3 is rhombohedral and silicon is diamond-cubic, thus resulting in complicated epitaxial relationships. Secondly, the lattice spacings are different by .perspectiveto.10%, although the exact bonding configuration at the Si/Al.sub.2 O.sub.3 interface at the deposition temperature is unknown. Thirdly, it contains aluminum, which under certain growth or processing conditions can become incorporated in the silicon film, thus doping it p-type.
Insulators other than sapphire have been used in the past as substrates for Si heteroepitaxy. Examples are spinel, MgOAl.sub.2 O.sub.3 ; beryllium oxide, BeO; and silicon carbide, .alpha.-SiC. However, no bulk insulator substrate other than sapphire has been commercialized and put to practical use for silicon dielectric isolation.
In an attempt to overcome the problems associated with using sapphire as a substrate for silicon, it has been suggested that yttria-stabilized cubic zirconia be utilized as a suitable substitute for sapphire.
Pure zirconia is monoclinic, but can be obtained in the cubic fluorite structure by adding a suitable additive, such as CaO or Y.sub.2 O.sub.3. Calcia-stabilized or yttria-stabilized zirconia (CSZ or YSZ, respectively) are defect solid solutions, in which the defects are oxygen vacancies, created to preserve lattice neutrality when Ca.sup.+2 or Y.sup.+3 ions are substituted for Zr.sup.+4 ions The cubic zirconias belong to a family of materials known as superionic conductors. These materials differ from insulators such as sapphire, in that (1) the transport of charge is effected by only one ionic species, O.sup.-- ions in the case of cubic zirconia, and not by both species, and (2) the conductivity is much higher reaching, at a sufficiently high temperature, values similar to those found for semiconductors.
At room temperature, however, YSZ is an excellent insulator. As a rule, the conductivity in the stabilized zirconias is highest for the smallest concentration of additive (yttria) needed to stabilize the cubic phase, and then decreases. The high oxygen ionic conductivity in YSZ signifies a high oxygen mobility and constitutes an added degree of freedom.
In the past, the use of cubic zirconia has been limited to its high-temperature ionic-oxygen-conducting properties, e.g., as an oxygen gauge or pump, often as part of an electrochemical cell. For such uses, polycrystalline material (bulk or thin film) was sufficient. Single-crystal material in relatively large size (up to 2 inches) and of high qaulity has become available recently through the use of cubic zirconia as a diamond substitute in the jewel industry.
In addition to its cubic crystal structure, stabilized zirconia is better lattice-matched to silicon than sapphire is. On the other hand, other properties of YSZ are less favorable than those of sapphire. Its thermal expansion coefficient is higher by about 30% than that of sapphire. Its thermal conductivity is very poor compared to that of Al.sub.2 O.sub.3, making YSZ more susceptible to, thermal shock, and limiting power dissipation through it. Its dielectric constant is higher than that of Al.sub.2 O.sub.3 resulting in potentially higher stray capacitance in integrated circuits; however, it's known that this problem can be minimized by proper design of circuit layout.
An additional practical drawback is that the as-grown YSZ crystals are randomly oriented, since at present they are obtained using a seedless process, called skull melting. In this technique, a specially shaped radio frequency apparatus is used for melting the source powders at .perspectiveto.2700.degree. C. As the melt is slowly lowered through the hot zone, spontaneous, random nucleation of single-crystal grains takes place in a skull of the same composition. Grains which are several inches long and 3-4 square inches in cross-section are obtained routinely. The grain size can be controlled by the growth conditions, and is related to the size of the skull. Sapphire, on the other hand, has a lower melting point of .perspectiveto.2050.degree. C. and is usually grown by the well known Czochralski technique using a seed of the desired orientation. To summarize, the main advantages of cubic zirconia over sapphire are its cubic structure, smaller lattice mismatch to silicon, and its high oxygen ion mobility at elevated temperatures.
The use of heteroepitaxial silicon deposition films on sapphire substrates as structural materials for use in fabricating integrated circuits has encountered some success. However, in using such films, it is a desirable objective to reduce the compressive stress and the concentration of crystallographic defects in the films, in order to increase the carrier mobilities and reduce the subthreshold leakage currents in the devices. In an attempt to achieve this objective, it has been found that silicon deposited films of very high quality can be produced by a method which utilizes the high mobility of oxidizing species in YSZ to oxidize the silicon side of the Si/YSZ interface, resulting in a dual-layer structure &lt;Si&gt;/amorphous SiO.sub.2 /&lt;YSZ&gt;. The method of this invention is unique to insulators that are oxygen superionic conductors, such as YSZ.
Accordingly, the primary object of this invention is to provide a novel method for treating silicon epitaxial films previously deposited onto insulating yttria-stabilized cubic zirconia substrates.
Another object of this invention is to provide a method for reducing the compressive stress and the concentration of crystallographic defects in silicon films previously deposited onto yttria-stabilized cubic zirconia substrates.
The above and still other objects and advantages of the present invention will become more readily apparent upon consideration of the following detailed description thereof.